We have a pre-fab fireplace (Heatilator) with glass doors. The fireplace extends into our family room with stone-faced walls built around it at the sides, and a raised stone hearth in front. In winter, ice cold air pours out the bottom of the fireplace, and the hearth is also cold as ice. I created an insulated cover for the front of the fireplace, which does a decent job of blocking the draft, but is a pain to take in and out when we want to use the fireplace. The fans in the unit are extremely noisy, so we never use them. Is there any kind of insulation I can stuff in the air space below the firebox to stop the drafts? Or will it just come out the top then? We had a similar fireplace at a previous house, and it didn't have cold-air drafts anywhere near what we have with this one.